In order to provide extremely wideband amplification, distributed amplifiers were developed which utilize traveling-wave concepts in the grid and plate circuits of a series of thermionic electron tubes. An input signal is applied to an appropriately terminated grid transmission line which consists of the grid-to-cathode capacitance of the tubes as shunting elements and the inductance between the tube grids as series elements. The output signal is obtained from an appropriately terminated plate transmission line consisting of the plate-to-cathode capacitance as shunting elements and the inductance between the tube plates as series elements. The input signal causes a wave to travel along the grid transmission line which, when it arrives at the respective grids of the electron tubes, produces current flow in the plate circuits of the tubes, resulting in waves that travel in both directions in the plate transmission line. For a perfect termination at the end of the plate line away from the output, the waves traveling toward such termination, which are out of phase, will be completely absorbed and will not contribute to the output signal. On the other hand, the waves traveling toward the output end of the plate line will add in phase, producing an output signal proportional to the number of tubes. Further details concerning early forms of distributed amplifiers may be found in the classic paper by E. L. Ginzton et al "Distributed Amplification", Proceedings of the I.R.E., August 1948, pages 956-969.
In recent years distributed amplifiers have been implemented at microwave frequencies using gallium arsenide field effect transistors as the active elements and microstrip transmission lines as the input and output lines. Microwave amplifiers of this type are described in detail in the papers by Y. Ayasli et al, "A Monolithic GaAs 1-13-GHz Traveling-Wave Amplifier", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-30, No. 7, July 1982, pages 976-981, and Y. Ayasli, "An Overview of Monolithic GaAs MESFET Traveling-Wave Amplifiers", International Journal of Electronics, 1985, Vol. 58, No. 4, pages 531-541.
Although distributed amplifiers fabricated with field effect transistors are smaller in size, consume less power and have less tendency for mechanical failure than distributed amplifiers constructed with thermionic electron tubes, nevertheless, both of the aforediscussed types of distributed amplifiers are limited in their operating frequencies by excessively large electrode capacitances and excessively long charge carrier transit times. In addition, the relatively low efficiency of field effect transistor distributed amplifiers gives rise to heat removal problems.
A further device of relevance to the present invention is the thin-film field emission cathode. This device comprises a metal/insulator/metal film sandwich with a cellular array of holes through the upper metal and insulator layers, leaving the edges of the upper metal layer (which serves as an accelerator electrode) effectively exposed to the upper surface of the lower metal layer (which serves as an emitter electrode). A number of conically-shaped electron emitter elements are mounted on the lower metal layer and extend upwardly therefrom such that their respective tips are located in respective holes in the upper metal layer. When appropriate voltages are applied between the emitter electrode, accelerator electrode, and an anode located above the accelerator electrode, electrons are caused to flow from the respective cone tips to the anode. Further details regarding thin-film field-emission cathodes may be found in the papers by C. A. Spindt, "A Thin-Film Field-Emission Cathode", Journal of Applied Physics, Vol. 39, No. 7, June 1968, pages 3504-3505, C. A. Spindt et al, "Physical Properties of Thin-Film Field Emission Cathodes with Molybdenum Cones", Journal of Applied Physics, Vol. 47, No. 12, December 1976, pages 5248-5263, and C. A. Spindt et al "Recent Progress in Low-Voltage Field-Emission Cathode Development", Journal de Physique, Vol. 45, No. C-9, December 1984, pages 269-278, and in U.S. Pat. No. 3,453,478 to K. R. Shoulders et al and U.S. Pat. Nos. 3,665,241 and 3,755,704 to C. A. Spindt et al.